Membrane Properties and Synaptic Currents Evoked in CA1 Interneuron Subtypes in Rat Hippocampal Slices. Morin, France, Clermont Beaulieu and Jean-Claude Lacaille. Centre de recherche en sciences neurologiques and D[acute]epartements de physiologie 1 et de pathologie 2 , Universit[acute]e de Montr[acute]eal, Montr[acute]eal, Qu[acute]ebec, Canada, H3C 3J7.
APStracts 3:0038N, 1996.
SUMMARY AND CONCLUSIONS
1. Intrinsic membrane properties and pharmacologically isolated excitatory and inhibitory postsynaptic currents were characterized using whole cell current- and voltage-clamp recordings, in combination with biocytin labelling, in different subtypes of CA1 interneurons and pyramidal cells in rat hippocampal slices. 2. Three classes of interneurons were selected based on their soma location in the CA1 region: (1) in stratum (str.) oriens near the alveus, (2) near str. pyramidale and (3) near the border of str. radiatum and lacunosum- moleculare. Each class of biocytin-labelled cells demonstrated specific cellular morphology. The somata of all interneurons were non-pyramidal in shape and usually multipolar. However, the pattern of dendritic and axonal arborizations of labelled interneurons differed in each class. 3. In current- clamp recordings, all interneuron subtypes had shorter duration and smaller amplitude action potentials than pyramidal cells. Fast and medium duration afterhyperpolarizations were larger in amplitude in interneurons. Cell input resistance was greater and [mu] embrane time constant was faster in all interneuron subtypes than in pyramidal cells. 4. Depolarizing current pulses evoked regular firing in all classes of interneurons, while burst firing was observed in 50% of pyramidal cells. With hyperpolarizing current pulses, all non-pyramidal and pyramidal cell types displayed inward rectification followed by anodal break excitation. 5. Electrical stimulation of nearby afferents evoked excitatory postsynaptic potentials (EPSPs) in all cells. EPSPs were of short duration and usually followed by inhibitory postsynaptic potentials (IPSPs). EPSPs were mediated by glutamate since they were blocked by non-N- methyl-D-aspartate (non-NMDA) and NMDA antagonists (6-cyano-7- nitroquinoxaline-2,3-dione (CNQX) and (+/-)-2-amino-5-phosphonopentanoic acid (AP5), respectively). In the presence of these antagonists, IPSPs were evoked in isolation and reversed near -72 mV. 6. In voltage-clamp recordings, non- NMDA EPSCs were isolated pharmacologically in the presence of AP5 and the GABA A antagonist bicuculline (BIC). Their properties were similar in all interneuron subtypes and pyramidal cells. I-V relations were linear, and mean reversal potentials were near 5 mV. Non-NMDA EPSCs were reversibly antagonized by CNQX. 7. NMDA EPSCs were pharmacologically isolated during CNQX and BIC application and were observed in all cell types. I-V relations of NMDA EPSCs demonstrated a region of negative slope at membrane potentials between -80 and -20 mV and their reversal potential was near 7 mV. The rise time of NMDA EPSCs was significantly slower in O/A interneurons than in other cell types. NMDA EPSCs were reversibly antagonized by AP5. 8. GABA A IPSCs were pharmacologically isolated in AP5 and CNQX and their properties were similar in all cell types. I-V relations of GABA A IPSCs were linear with mean reversal potentials near -32 mV. GABA A IPSCs were reversibly blocked by BIC. 9. In conclusion, morphologically different subtypes of interneurons located in O/A, near PYR and in L-M, displayed intrinsic membrane properties that were distinct from pyramidal cells, but were similar among them. In contrast, the properties of non-NMDA, NMDA and GABA A postsynaptic currents were similar between interneurons and pyramidal cells, except for NMDA EPSCs which had slower rise times in O/A interneurons. 

Received 26 May 1995; accepted in final form 1 February 1996.
APS Manuscript Number J345-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 14 February 96